This invention relates to a circuit arrangement for subscriber lines using two symmetrical line pairs for additional message in a telecommunication system. The invention has been found to be particularly useful in video telephone applications.
As is known, line amplifiers in transmission lines may be remotely supplied in series for additional character transmission or the like. The remote-supply circuit is generally constructed as a phantom circuit of the two line pairs.
The remote supply to intermediate amplifiers may be constructed, for example, as described in the article "Baseband Video Transmission on Loops and Short-Haul Trunk," Bell System Technical Journal, February 1971, pages 395 through 425. If comparatively long subscriber lines are employed, it is necessary to insert frequency-correcting intermediate amplifiers into the symmetrical line pairs over which the video information is transmitted. To avoid the need for additional conductors for feeding the amplifiers, a direct-current source is connected as a phantom circuit to both line pairs such that first the amplifiers of one line pair and subsequently, those of the other line pair are fed simultaneously with the required portion of the applied voltage on the return path.
In addition to the two symmetrical line pairs for directional message transmission, hereinafter referred to as video transmission but not restricted thereto, a further circuit for the telephone set is required for such a subscriber line. Thus, a total of six wires is required.
An object of this invention is to provide a line construction which will permit the number of required lines to be reduced in such circuits.